The present invention relates to an attachment structure of a base and a top cover in a storage device such as magnetic disk unit and magneto-optical unit, and more particularly relates to an attachment structure which prevents deformation of the base or the top cover due to a temperature change.
In the magnetic disk unit, a mechanical apparatus is constituted as a head disk assembly (hereafter, mentioned as HDA in the specification). This HDA is constituted by the base, a magnetic disk having been accommodated in the base, a head support mechanism, a spindle motor and electronic components etc., and by the top cover which forms a sealed structure by being joined to the base in order to form an enclosure to provide a clean air environment. In order to make the HDA into a sealed state, the top cover is screwed to the base.
In the head support mechanism of this magnetic disk unit, a head/slider performs reading/writing of data with respect to the rotating magnetic disk, and an actuator positions the head to a predetermined position. A drive mechanism of this actuator is a voice coil motor (hereinafter, mentioned as VCM in the specification) constituted by a voice coil, a magnet and a yoke.
In the head support mechanism of the magnetic disk unit like this, if the magnetic disk rotates, the slider having an air bearing surface (ABS) slightly lifts from a magnetic disk surface. The head can perform the reading/writing of the data in a predetermined position of the disk surface by rotating, by a driving force of the VCM, the slider about a pivot shaft in an approximately radial direction of the magnetic disk under a state that the slider has slightly floated from the magnetic disk surface. The positioning of the head is performed by the fact that the head reads a servo data having been previously recorded in the magnetic disk and this information is fed back to the actuator. If the head is positioned, a recording or regeneration of the information to or from a recording or recorded face of the magnetic disk is performed.
For the above magnetic disk unit, in a case where the materials of the base and the top cover (typically metals) are different and have different thermal expansion coefficients, a bimetal effect occurs depending on the use environment temperature. FIG. 4 shows a deformation state of the HDA in which this bimetal effect has occurred. In an HDA 100, the material of a base 101 is aluminum, and the material of a top cover 102 is a stainless steel whose thermal expansion coefficient is smaller than that of the aluminum. Further, FIG. 4(A) is a drawing showing an aspect in which the base 101 of the aluminum deforms under a high temperature environment, and FIG. 4(B) is a drawing showing an aspect in which the base 101 of the aluminum deforms under a low temperature environment.
In this HDA 100, the top cover 102 is fixed to the base 101 by screws 103, and the base 101 of the aluminum becomes larger in its thermal expansion coefficient than the top cover 102 of the stainless steel. Accordingly, under the high temperature environment, a bottom part 101a of the base 101 deforms so as to bend downward as shown in FIG. 4(A) and, under the low temperature environment, the bottom part 101a of the base 101 deforms so as to bend upward as shown in FIG. 4(B).
If the bottom part 101a of the base 101 deforms so as to bend downward and/or upward, a positional relation between a magnetic disk 104 and a head (not shown in the drawing) having been provided in a slider 106 of an HSA (head stack assembly) 105 deviates. This is because the magnetic disk 104 is fixed to a spindle hub of a spindle motor 107 which is provided in the bottom part 101a of the base 101, and the head support mechanism 105 is rotatably fixed to a pivot shaft 109 which is provided in the bottom part 101a of the base 101 through a carriage 108.
In a case where the base 101 and the top cover 102 deform while expanding or contracting by the same amount, no problem occurs in an operation mechanism inside the HDA. However, if the base deforms under a mode having been shown in FIG. 4 and thus the positional relation between the magnetic disk 104 and the head of the head support mechanism 105 deviates, an offtrack error occurs or a floating posture of the slider 106 of the head support mechanism 105 changes. The offtrack error refers to the situation in which, when the head support mechanism is mounting plural heads/sliders, a track to which a certain head corresponds when the other head has been positioned to a specified cylinder deviates before and after a temperature change.
If the offtrack error occurs in the head/slider, it becomes difficult to write the data to a right track or regenerate the data from a purposive track. Further, if the floating posture of the head/slider changes, a flying height of the head changes, so that a dysfunction arises in the recording and the regeneration of the data. There are various proposed techniques for preventing the bimetal effect by the base and the top cover. See, e.g., JP-UM-A-2-101392 Gazette, JP-A-6-162758 Gazette, and International Laid-Open 96/17349 Pamphlet.